Natriuretic peptides (NPs) regulate blood pressure and fluid balance via their diuretic, natriuretic, and vasoactive properties, and are biomarkers of heart failure in adults and children. Infants with patent ductus arteriosus (PDA) have increased ANP and BNP levels due to cardiac overload. NP levels normalize soon after PDA treatment. These findings suggest that NPs primarily reflect cardiac status, but we speculate that ongoing elevation of NPs in the presence of PDA could augment DA relaxation and create a feed-forward mechanism to further inhibit DA closure. Our observation that NPs induce DA dilation prompts further speculation that NP administration may be a novel approach to maintain patency of the postnatal DA. Little information exists on the role of NPs in DA development or as mediators of DA tone. Our preliminary data show that the Npr1, Npr2, and Npr3 receptors are differentially expressed in the fetal and newborn mouse DA. Exposure of the isolated fetal mouse DA to different NPs produced dose-dependent dilation to CNP>ANP>BNP under fetal and newborn oxygen conditions. Moreover, mice with PDA have elevated circulating NP levels. We hypothesize that: 1) NPs mediate relaxation of the DA and may prolong patency of the postnatal DA, and 2) elevated postnatal NP levels due to PDA and secondary congestive heart failure help to maintain the DA in a non-contractile state. This hypothesis will be examined in three specific aims. Aim 1 will determine the expression pattern of NPs and their receptors in the fetal and newborn DA, in mouse models of PDA, and cultured DA cells. Aim 2 will determine, in vitro, NP-induced changes in the tone of DAs from: wild type (preterm vs. term) mice, wild type mice treated with NP antagonists, Npr1 (-/-) and Npr2 (-/-) mice, mice with a PDA phenotype (COX-1(-/-) /COX-2(-/-) and COX-inhibited mice), and the closed DA of postnatal P1 and P2 mice. cGMP production and receptor affinity will be determined. Aim 3 will determine, in vivo: the response of term newborns exposed to NPs (to prevent postnatal closure); the response to fetal exposure to NP receptor antagonists (to induce fetal DA constriction); and the effect of NP antagonists on mouse models of PDA (to induce PDA constriction in neonates). These studies will provide new information on novel roles for NPs as DA mediators and identify new potential therapeutic targets for modulation of DA tone.